This invention relates to an improved process for dyeing fibers with indigo dye, including natural cellulose fibers, textile fabric and yarn. The improved indigo dye process includes premordanting the fibers and oxidizing the leuco indigo dye by flooding the fibers with cold water, thus eliminating the requirement for skying and polluting effluents. Further, the indigo dye process of this invention can be adapted to utilize conventional garment, package and piece dye machinery and processes.
Indigo has been used to dye fabric with "indigo blue" since before recorded history. The sap which oozes from the plant when bruised was applied to fabric by ancient Egyptians, Greeks and Romans. Indigo has been used in India to dye fabric for at least 4,000 years by methods which are practically identical to the methods employed today. Indigo was introduced in Europe in large quantities by the Dutch East India Company in the early 17th century.
Indigotin (C.sub.16 H.sub.10 N.sub.2 O.sub.2) is the true coloring matter of indigo. When pure, indigotin forms a dark, rich blue powder or bronzy blue-colored needle crystals. The most important reaction of indigotin is its reaction with reducing agents. When subjected to a reducing agent in the presence of alkali, indigotin combines with two atoms of hydrogen and is reduced to a colorless body, known as indigo-white or the leuco form, which is insoluble in water, but dissolves in alkali, with a yellow color. This reaction may be represented, as follows: ##STR1## It is reoxidized to indigotin with great ease, simply by exposure to oxygen, by the following equation: EQU C.sub.16 H.sub.12 N.sub.2 O.sub.2 +0=C.sub.16 H.sub.10 N.sub.2 O.sub.2 H.sub.2 O
The commercial vat method thus utilizes these reactions to dye fabric or yarn indigo blue. Commercial indigo vat dyeing is carried out in an aqueous alkali vat containing the reduced leuco form of indigo. Fibers in the form of warp yarns are dipped into the vat for a residence time sufficient to permit the fibers to absorb the desired amount of leuco dye. Following each dip, the fibers are squeezed between rolls and then carried into the open air, which is known as "skying," wherein the leuco dye is oxidized to the insoluble indigotin form in the fibers.
Oxidation of the reduced leuco form in the dip vat is troublesome in all vat dyeing, but is a particular problem with indigo dyeing. The oxidation occurs where the dye liquor contacts the air, especially in the region of the squeeze rolls where multiple interfaces are generated, exposing large areas of leuco dye liquor to the air as the dye liquor drains from the squeeze rollers. This oxidation results in a significant loss of dye liquor and formation of floating scum of oxidized insoluble dye. U.S. Pat. No. 4,283,198 discloses an "inert atmosphere" indigo dye process which includes an enclosure of the air-liquor interface of a leuco indigo bath comprising sodium hydrosulfite and sodium hydroxide, which results in consumption of oxygen at the enclosed interface. However, the enclosure cannot be gas tight and would not result in an inert gas enclosed process. Further, the process is impractical for commercial indigo dyeing operations and thus does not solve the problems associated with the use of natural indigo dyes.
Synthetic indigo has essentially replaced natural indigo in commercial dyeing. The production of synthetic indigo requires the use of toxic materials, including sodium cyanide, formaldehyde, sodium and potassium hydroxide and synthetically produced aniline, which are carried through to the cloth. The waste products include the unspent toxic chemicals described above, plus sulfuric and hydrochloric acid, sodium dioxide and insoluble salts. Thus, there has been a renewed interest in the use of natural indigo; however, the prior indigo dye methods result in dyed fibers or fabric which are not nearly as wash and lightfast as synthetic dyes and naturally dyed colors are difficult to reproduce using known techniques.
Thus, there remains an important need for a process for dyeing fibers, particularly cellulose fibers, which is suitable for natural indigo and which produces a consistent natural color, which is wash and lightfast. The process for dyeing textile fibers with indigo dye of this invention is suitable for natural indigo, thus avoiding the problems associated with synthetic indigo dyes, including toxic wastes. The process of this invention may also be utilized to dye over other colors, producing a full palette of consistently reproducible natural colors which are wash and lightfast.